The disclosure in general relates to turbomachines and impellers thereof. Embodiments disclosed herein refer to so-called unshrouded impellers.
Radial or mixed turbomachines usually comprise one or more impellers arranged for rotation in a casing. Each impeller is comprised of a hub and a plurality of blades. The blades extend from a blade root, at the front surface of the hub, to a blade tip.
Shrouded impellers are known, wherein blades are arranged between the hub and an outer shroud surrounding the hub and rotating therewith. Closed flow vanes between the shroud, the hub and neighboring impellers are thus defined. The shroud improves the stiffness of the impeller blades.
Some turbomachines use unshrouded impellers, wherein the blades extend from the hub and end at respective free blade tips. The flow vanes are in this case defined between the hub, pairs of neighboring blades and a stationary surface of the casing.
As other mechanical components, rotating turbomachine impellers are subject to resonance phenomena. When a turbomachine impeller rotates at a rotational speed proximate a resonance frequency, the blades of the impeller can experience modal displacements, which induce machine vibrations and fatigue stress, eventually resulting in turbomachine failure. Different parts of a machine or machine component can be subject to different modal displacements at the same frequency, i.e. different areas of the machine or machine component undergo different displacements when the machine or component is subject to vibration at a given resonance frequency.
While shrouded impellers are less subject to dynamic stresses resulting from resonance phenomena, unshrouded impellers are more subject to deformations when operating near a resonance frequency. In particular, in case of large flow coefficients, controlling the aeromechanical behavior of impellers becomes difficult. The first flexural mode frequency of this kind of impellers is relatively low and may be easily excited under normal operating conditions of the turbomachine.
In order to at least partly alleviate the above problem, tapered blades are often used, i.e. blades the thickness whereof reduces from the blade root towards the blade tip. This approach, however, is unsatisfactory, because the first flexural natural frequency of the blade cannot be increased enough to move sufficiently away the vibration mode from the impeller operating range.
Therefore, there is still a need of improving radial or mixed-flow turbomachines using unshrouded impellers, in order to reduce the above mentioned drawbacks connected to resonance frequencies.